Substantial reserves of hydrocarbons (e.g. oil) are known to exist in reservoirs which have very low permeabilities. For example, billions of barrels of oil of proven reserves are known to be trapped in diatomaceous reservoirs in California, alone. A diatomaceous reservoir (i.e. formed primarily of diatomite) is characterized by high porosity, high compressibility, and very low permeability (e.g. as low as 0.1 millidarcy) which makes the recovery of the oil from these reservoirs extremely difficult.
Several methods have been proposed and/or used for producing these low permeability reservoirs. For example, routine, secondary-production techniques (e.g. water and/or gas floods, steam stimulation, etc.) are often used but due to the low permeability and the absence of any substantial natural fracture network in diatomaceous reservoirs, it is difficult to establish the necessary flow of the drive fluid through the reservoir. Of course, these reservoirs may be hydraulically fractured to improve the permeabilities thereof. However, due to the subsidence/compaction characteristics of diatomaceous reservoirs, the hydraulically-induced fractures along with the natural fractures have a tendency to close as fluids are withdrawn from the reservoir, thereby again substantially decreasing the permeability of the formation long before the recovery operation is completed.
Another technique for producing low permeable reservoirs is one which is known as "imbibition". In an imbibition waterflood, the natural or induced fracture network in the reservoir is flooded with water but, unlike a conventional waterflood, there is no co-current flow of water and oil through the rock matrix. In other words, the water does not push the oil ahead of it so there is no flow of oil and water through the formation in the same direction. Instead, capillary action causes water in the fractures to soak or imbibe into the matrix through the fracture face.
The oil displaced by this water, in turn, flows from the matrix into the fracture through the same fracture face by means of countercurrent flow. The displaced or exchanged oil is then produced from the fracture network by excess water flowing therethrough. For a further description and discussion of "imbibition", see U.S. Pat. No. 3,490,527, incorporated herein by reference. Recently, an imbibition process carried out in a specialized fracturing pattern has been proposed for increasing the production from diatomaceous reservoirs, see commonly-assigned, U.S. patent application Ser. No. 08/142,028, filed Oct. 28, 1993now U.S. Pat. No. 5,3777,756.
Further, cyclic injection of steam has been used for the recovery of heavy oil. However, it has usually been used in formations that are generally unconsolidated and having high permeabilities since it is difficult for the steam to penetrate any substantial distances into low permeable reservoirs such as those formed of diatomite. Further, where there is extremely viscous oil in some unconsolidated formations, high pressure steam has been used to fracture the formation to increase the rate of heat input into the reservoir, see "STEAM STIMULATION HEAVY OIL RECOVERY AT COLD LAKE, ALBERTA", R. S. Buckles, SPE 7994, Ventura, Calif., Apr. 18-20, 1979. However, in these known steam recovery operations, imbibition is not an important recovery mechanism.